为航天器热设计与质量控制所设计的便携式发射度测量系统

Design of a Portable Emittance Measurement Systemfor Spacecraft Thermal Design and Quality ControlH. Yamana1, S. Katsuki2, A. Ohnishi3, 5 and Y. Nagasaka41 School of Integrated Design Engineering, Keio University, 3-14-1, Hiyoshi, Y okohama, 223-8522, Japan.2 Specialty Chemicals & Products, Aerospace Materials, UBE Industries, LTD., 1978-10, Kogushi, Ube, Yamaguchi, 755-8633, Japan.3 Institute of Space and Astronautical Science (ISAS), Japan Aerospace and Exploration Agency (JAXA), 3-1-1, Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan.4 Department of System Design Engineering, Keio University, 3-14-1, Hiyoshi, Yokohama, 223-8522, Japan.5 To whom correspondence should be addressed. E-mail: ohnishi@isas.jaxa.jpABSTRACTThis paper reports the development of a portable hemispherical total emittance εH measurement system for spacecraft thermal design and quality control. The measurement principle is the reflective method, and the measurement system consists of a blackbody furnace, an integrating sphere, a thermopile and so on. A prototype εH measurement system was constructed and evaluated by comparing the measurement data with those obtained by the calorimetric method. From the evaluation results, it was confirmed that the measurement accuracy of this system is ±0.05. The systematic errors caused by the deviance from the restrictions of Kirchhoff’s law and by the characteristic of the integrating sphere were estimated and reduced or compensated. The prototype εH measurement system was improved for a portable system with higher measurement accuracy and higher measurement speed. To evaluate the validity of the portable system, the εH of variable thermal control materials were measured. The results by the portable system agreed well with those obtained by the calorimetric method.KEY WORDS: hemispherical total emittance, on-site measurement, portable system, quality control, reflective method,thermal control material, spacecraft thermal design.1. INTRODUCTIONThe accurate data of hemispherical total emittance εH of thermal control materials are required for thermal design of spacecraft. For more than two decades, our laboratory has developed several emittance measurement systems [1] which can be used in the laboratory for various purposes. Generally, for spacecraft thermal design, the εH of thermal control materials are referred to the values of the database [2-3] which was built by using these emittance measurement systems, and these data are considered to be constant before launch. However, there are possibilities that the values of εH of materials actually used for thermal design differ from those of the database, and that the qualities of thermal control materials change. The quality difference and change of the spacecraft materials cause critical problems in space flight mission, but on-site measurement before launch by using conventional measurement systems is difficult because of their large sizes or low accuracies. We have proposed to develop a portable εH measurement system with high accuracy, high measurement speed, light weight, and suitable for the following purposes.1.On-site measurement of εH change during assembling and preserving the spacecraftson the ground.2.Quality control in material fabricating for material research and development.As a first step of this study, a prototype εH measurement system has been developed and evaluated. This paper describes the details of prototype system and measurement results of emittance for various spacecraft materials. In addition, a portable εH measurement system for which the prototype system was improved and evaluation results are discussed.2. PRINCIPLEThe measurement principle is the reflective method. From the law of energy conservation, the relation between the directional spectral absorptanceα′ and theλρ′ for an opaque body is expressed as directional/hemispherical spectral reflectanceλ()()1,,,,,,=′+′sample sample T T φθλρφθλαλλ, (1)where λ is the wavelength of the incident light, θ is the incident angle, φ is the azimuth angle, and T sample is the temperature of the sample. Kirchhoff’s law for hemispherical total properties are expressed as()()sample H sample H T T αε=,(2)and be applied to give ()()1=+sample HH sample H T T ρε. (3) Eq. (3) is satisfied when either of the following restrictions is applied [4].1. The incident radiation is independent of angle and has a spectral distributionproportional to that of a blackbody at T sample .2. The incident radiation is independent of angle, and the sample has directional-graysurface.3. The incident radiation from each direction has spectral distribution proportional tothat of a blackbody at T sample , and the sample has diffuse-spectral surface.4. The sample has diffuse-gray surface.However, it is difficult to fulfill these restrictions and to measure the accurate hemispherical/hemispherical total reflectance ρHH by a portable system. Thus, we used an integrating sphere to detect the directional/hemispherical reflective intensity from a sample. The advantage in using the integrating sphere is that the directional/hemispherical reflectance ρDH can be treated equal to the hemispherical/hemispherical reflectance ρHH [5]. In addition, when the light source has a spectral distribution approximately proportional to that of a blackbody over wide wavelength range, and the temperature difference between the sample T sample and the light source T light is small, εH of the sample can be obtained by Eq. (3).3. PROTOTYPE MEASUREMENT SYSTEM3.1. System ComponentsThe schematic diagram of the prototype hemispherical total emittance εHFigure 1 Schematic diagram of prototype εH measurement system. ParabolicMirror Blackbody FurnaceShutterDC PowerIntegrating SpherePCThermopile MultimeterSample or Reference Amplifiermeasurement system is shown in Figure 1. This system consists of a blackbody furnace, a parabolic mirror coated with gold, an integrating sphere with a shutter, a thermopile (KRS-5 window) with an amplifier, and a data acquisition system. The incident light from the blackbody furnace irradiates a sample at an angle of 7 degrees in order to minimize the specular reflective energy loss [6]. The reflected light from the sample multiple reflects inside the integrating sphere, and the multiple reflected light is detected by the thermopile over the wavelength region from 0.6 to 42.0 µm, which covers 95 % of the total emissive power of the ideal blackbody at 300 K. The relation between the input infrared power and the output voltage of the thermopile with the amplifier has linearity, thus the output voltage is expressed as a linear function of the εH of the sample (see 3.2.).The spectral characteristic of the light source is the most important in the reflective method. The design of the light source requires the following performances:1. covers spectral distribution proportional to that of a blackbody and has high emittance over wide wavelength range2. low power supply control3. small size and light weightThe blackbody furnace which satisfies the above conditions was designed and constructed. The core of the blackbody furnace is cylindrical structure made of carbon,whose size is 9 mm in diameter and 45 mm length. The inner surface of the core is coated with black paint, which has spectral emittance of 0.94 in the wavelength region from 2 to 25 µm. Tantalum wire is coiled around the core, and the blackbody furnace is controlled at 450 K loading 8.4 W. When the temperature of the inner surface of the core is constant, the effective emittance εe of the blackbody furnace is expressed as [7] ()()[]()A a A a f A a bw bw bw e +−−−+=111εεεε, (4) ()[]2211black r D f += and ()]black r D A a 2121+=, where εbw is the emittance of the inside surface, D is the depth and r black is the diameter of the aperture of the core. From Eq. (4), the εe of the blackbody furnace used in the present work is more than 0.99.The integrating sphere is made of aluminum alloy and the inside surface is coated with gold. The inside diameter is 30 mm and it has three ports, the first-port (6 mm in diameter) is used for the input of light from the source, the second-port (6 mm in diameter) is used for setting the sample, and the third-port (4 mm in diameter) is used for the output of the reflected light from the sample into the detector.3.2. Measurement ProcedureThe detected energy of the thermopile E s 0 when the shutter is closed can be expressed as()()i I i s I s s E E E εε,,0+=, (5) where E s,I (εs ), E i,I (εi ) are emissive energy of the sample and the integrating sphere. On the other hand, the detected energy of the thermopile E s 1 when the shutter is opened can be expressed as()()()s R s i I i s I s s E E E E εεε,,,1++=, (6) where E s,R (εs ) is reflective energy of the sample. E s,R (εs ) is obtained by taking the difference between E s0 and E s1:()s R s s s E E E ε,01=−. (7)The intensity difference of voltage between the conditions when the shutter is closed and opened is represented as a linear function of E s,R (εs ). Therefore, by measuring temperatures and reflective intensities of two reference samples with known temperature dependence of low and high emittance, the calibration line is obtained. The emittance of the sample is obtained by measuring its reflective intensity:()()(){}()()h l h l l l h h s h h l l s s V V V T V T V T T T −−+−=εεεεε, (8)where ε is the hemispherical total emittance, T is the temperature, V is the intensity difference of voltage when the shutter is closed and opened, and the subscripts are the sample (s ) and the reference samples with low emittance (l ) and with high emittance (h ).3.3. Emittance Evaluation ResultsThe hemispherical total emittance of spacecraft materials were measured by the prototype εH measurement system and the calorimetric method. The measurement accuracy of the calorimetric method is 2.0 %, and a detailed explanation of this system is described in Ref. 1 and 3. To evaluate prototype system, aluminum coated polyimide (25 UPILEX-R/Al), Highly Oriented Graphite Sheet (HOGS) [8], and germanium coated-0.5-0.4-0.3-0.2-0.100.1I n t e n s i t y , V Time, s Figure 2 The time fluctuation in the intensities of the detector by prototype system.Table 1 Measurement results of emittance by prototype system.Measurement results of εHSample Present work Calorimetric method ∆εH25 UPILEX-R/Al 0.54 0.57 [2] -0.03HOGS 0.33 0.29 [8] +0.04 Ge/50RN/Al 0.64 0.69 -0.05aluminized polyimide (Ge/50RN/Al) were used as test samples. Aluminum deposited film (Al: εH = 0.05 @ 293 K) of low emittance and electrically conductive black polyimide (Black Kapton: εH = 0.80 @ 293 K) of high emittance were used as reference samples. In the present study, the reflective intensities of the samples were measured at room temperature and the emittance of the reference samples were obtained from data at 293 K. The time fluctuation in intensities of voltage of detector is shown in Figure 2 and the measurement results of emittance is shown in Table 1. In all samples, the results of the prototype system agreed within 0.05 with those of the calorimetric method.4. ESTIMATION OF SYSTEMATIC ERRORS4.1. Temperature Difference between Sample and Light SourceIn the reflective method, the incident light must have a spectral distribution proportional to that of a blackbody at sample temperature T sample because of the restrictions of Kirchhoff’s law. However, in the present prototype measurement system, the blackbody furnace as a light source is used at 450 K, and T sample is at room temperature. This difference between light source temperature T light and T sample (∆T ) causes systematic error. This systematic error ∆α is the difference between hemispherical total emittance and the hemispherical total absorptance of the sample, and it can be expressed as ()()(){}()()(){}(())∫∫∫∫⋅−−⋅−=−=∆λλλλλρλλλλλραααλλλλd T i d T i T d T i d T i T T T T T sample b sample b sample light b light b sample sample sample light sample ,,,1,,,1,,.(9)00.010.020.030.040.05∆α∆T 00.10.20.3∆α (M A X )∆T )Figure 3 The systematic error ∆α, which is the difference between hemispherical total emittance and the total hemispherical absorptance of the sample, versus the temperature difference between T light and T sample (∆T ) of Al, Black Kapton, 25 UPILEX-R/Al, HOGS (a) and imaginary assumed material which brings the error maximum (b) when sample temperature T sample is 300K.The values of ∆α of Al, Black Kapton, 25 UPILEX-R/Al and HOGS were estimated when sample temperature T sample is 300K. The spectral reflectance ρ(λ,T sample ) of the samples were measured by using the Fourier Transform Spectroscopy (Bio-Rad: FTS-60A/896) in the wavelength range from 1.6 to 100 µm, and the spectral emissive power of the blackbody i b λ(λ, T ) was calculated from Planck’s law. In addition, ∆α of the imaginary assumed material which brings the error maximum was estimated. The spectral reflectance of the imaginary assumed material ρim (λ) can be expressed as()()([]()([⎩⎨⎧≤>=samplelight samplelight im T i T i T i T i ,','0,','1λλλλλρ)], (10) where i’(λ, T ) are obtained by normalizing i b λ(λ, T ).∆α versus ∆T are shown in Figure 3. When light source temperature T light is 450K, the systematic error for spacecraft materials are less than 0.05. We have worked to reduce this error and were successful in conducting measurements at ∆T = 60K by a portable system hereinafter described (see 5.). As a result, this systematic error for these materials is less than 0.02.4.2. Characteristic of Integrating SphereThe ideal measurement relationship is for the ratio of radiance produced inside the sphere to be equal to the ratio of the reflectance for each material [9]:r s r s I I ρρ=. (11)Where I is the reflective intensity, ρ is the reflectance of the sample, and the subscripts are the sample (s ) and the reference sample (r ). However, the average reflectance of the sphere changes when the sample is substituted for the reference sample. The precise measurement equation for a substitution sphere is expressed ass r r s r s I I ρρρρ−−⋅=11, (12)()w pp p w w S S S S ∑∑+−=ρρρ, (13) where ρ is the average reflectance for the entire integrating sphere, S is the area, and the subscripts are the sphere surface (w ) and the sphere ports (p ).Figure 4 shows the relationship between the measurement reflectance and the actual reflectance in the present work by Eqs. (11) and (12) when the sphere surface and-0.03-0.02-0.0100.010.020.0300.20.40.60.81(M e a s u r e d -A c t u a l ) R e f l e c t a n c e o f S a m p l e Actual Reflectance of SampleFigure 4 The difference between the measurement reflectance and the actual reflectance in the present work when the sphere surface and samples are ideal diffuser.samples are ideal diffuser.The reflectance of the sphere surface ρw which was measured by using the FT-IR in the wavelength range from 1.6 to 100 µm is 0.93, and the reflectance of the port for input light and detector are 0. From this calculation result, the systematic error caused by the characteristic of the integrating sphere is less than ±0.03. This error is compensated in the portable system hereinafter described (see 5.) and can be reduced by increasing calibration points, decreasing the areas of the sphere ports and improving the reflective characteristic of the sphere surface.5. PORTABLE SYSTEMWe miniaturized and divided the present prototype measurement system into a measurement unit and an operating unit. The appearance and schematic diagram of a portable system are shown in Figures 5 and 6. The measurement unit consists of a blackbody furnace, a parabolic mirror, an integrating sphere with a shutter, a thermopile, three thermo-sensors and temperature control unit. The thermo-senosor1, 2 and 3 measure blackbody furnace temperature T light, room temperature T room, and sample temperature T sample respectively. By employing the temperature control unit and the thermo-sensor1 and 2, T light can be controlled at desired temperature (T room ~ 363K). By introducing the thermo-sensor3, the accurate εH of reference samples from knownOperating Unit Measurement UnitFigure 5 The appearance of the portable hemispherical total emittance measurement system.Integrating Sphere Temperature Control UnitThermo-Sensor 1Thermopile Blackbody FurnaceThermo-Sensor 2Control Unit Thermo-Sensor 3PreamplifierShutter LCD Operation ButtonPower Supply UnitT sampleT lightT roomMeasurement UnitOperating UnitParabolic MirrorFigure 6 The schematic diagram of the portable εH measurement system.temperature dependence data, and accurate calibration line could be obtained. The operating unit has a control unit with memories for data calculation and save, a power supply unit and a LCD display. The dimensions of the measurement unit are 88 mm by 134 mm by 78 mm, and the total weight is less than 2 kg. The measurement is completed within one to two minutes.The hemispherical total emittance of various spacecraft thermal controlmaterials were measured by using the portable system at room temperature. In this measurement, temperature of the blackbody furnace was controlled at 363K. Figure 7 shows the emittance results measured by the portable system versus by the calorimetric method. Only for 7 UPILEX-S/Al and 12 UPILEX-S/Al, the measurement results were compared with the calculation values in Ref. 10. The measurement results differences between the portable system and the calorimetric method were +0.03 and +0.07 for 25 UPILEX-R/Al and HOGS respectively, while these difference between the prototype system described in chapter 3 and the calorimetric method were -0.03 and +0.04. Although a slight deviation can be seen for low emissive materials, the over all results of the present portable system agreed well with the calorimetric method.0.20.40.60.81Emittance Measured by Calorimetric MethodE m i t t a n c e M e a s u r e d b y P o r t a b l e S y s t e mFigure 7 The comparison of measured εH conducted using the portable system and the calorimetric method for various thermal control materials.6. CONCLUSIONSThe development of a portable hemispherical total emittance εH measurementsystem was presented in this paper. The principle of this system was reflective method using the integrating sphere and the blackbody furnace. The systematic errors caused by the temperature difference between sample and light source and by the characteristic of the integrating sphere were estimated. To evaluate the portable εH measurement system, hemispherical total emittance for various spacecraft thermal control materials were measured and were compared with those obtained by the calorimetric method. For almost all materials, the results obtained by the portable system agreed well with those obtained by the calorimetric method.ACKNOWLEDGMENTSWe would like to thank S. Tachikawa of ISAS/JAXA, Ph.D. H. Nagano of KeioUniversity and J. Kimura of JST Corporation for their exciting discussions. Professor T. Makino of Kyoto University is gratefully acknowledged for helpful suggestions.REFERENCES1. A. Ohnishi, T. Hayashi and H. Nagano, 4th Japan Symposium on ThermophysicalProperties, (1983), pp.1-4. (in Japanese)2.JSTP, Handbook of Thermophysical properties, (YOKENDO, Tokyo, 1990),pp.324-325. (in Japanese)3. A. Ohnishi, ISAS RESEARCH NOTE, 113 (ISAS, 2000). (in Japanese)4.R. Siegel and J. R. Howell, Thermal Radiation Heat Transfer 4th Edition, Taylor &Francis, (2002), pp.1-69.5. A. Parretta, H. Yakubu and F. Ferrazza, Optics Communications, 194:17 (2001).6. A. Ohnishi and T. Hayashi, Proc. Int. Symp., Toulouse, France, (1983), pp.467-470.7. A. Gouffe, Rev. Opt., 24:1 (1945).8.H. Nagano, A. Ohnishi and Y. Nagasaka, Journal of Thermophysiscs and HeatTransfer, 15:347 (2001).9. F. Grum and T. E. Wightman, Appl. Opt., 16:2775 (1977).10.K. Fukuzawa, A. Ohnishi and Y. Nagasaka, Transactions of the Japan Society forAeronautical and Space Sciences, 50:129 (2002).。